A generally employed process for wet development in electrophotography comprises charging a photoreceptor, image-wise exposing the charged photoreceptor to form an electrostatic latent image, developing the latent image with a liquid developer which is a dispersion of toner particles consisting mainly of a resin and a colorant usually in an aliphatic hydrocarbon, and then transferring and fixing the obtained toner image to receiving paper to form an image. In this process, when a photosensitive paper or film coated with a photoconductive material, e.g., zinc oxide, is used as a photoreceptor, the transfer step may be omitted to directly fix the developed toner image to the photoreceptor. Wet development is frequently utilized also as a developing means in other recording processes including electrostatic recording, in which an electrostatic latent image is formed on a dielectric by means of electrical input without using a photoreceptor.
In wet development, a dispersion of fine toner particles of from a submicron size to about several micrometers in a high-electrical-resistivity carrier liquid, e.g., an aliphatic hydrocarbon as mentioned above, is used to develop a latent image mainly by means of electrophoresis. This technique therefore has an advantage that images having high resolution are more easily obtained than dry development with toner particles of several micrometers or larger.
However, since the liquid developer having a submicron toner particle diameter is reduced in charge amount per toner particle, the rate of electrophoretic development with the developer is low. As a result, the conventional liquid developer has problems that (1) an increased image density is not obtained and (2) the liquid developer is unsuitable for high-speed copying. In addition, most of the toners investigated so far have drawbacks that they show poor adherence to substrates because fixing thereof is based on the drying, solidification, and adhesion of the resin dissolved in the carrier liquid, and that the images formed therefrom have insufficient mechanical strength because the cohesive force of the image parts themselves is low.
To mitigate these drawbacks, a liquid developer comprising an aliphatic hydrocarbon and dispersed therein toner particles consisting mainly of a thermoplastic resin and a colorant and having an average particle diameter of several micrometers has been proposed as described in JP-A-8-2851, JP-A-58-152258, JP-A-59-87463, and U.S. Pat. No. 4,794,651. (The term "JP-A" as used herein means an "unexamined published Japanese patent application.")
However, the proposed liquid developer, which is a mere dispersion in an aliphatic hydrocarbon of toner particles containing a thermoplastic resin, has a drawback that when a latent image formed on an electrophotographic photoreceptor is developed with the liquid developer by a known method for liquid development, an image of low quality with low image density and low resolution is obtained in most cases or no image is obtained in some cases, although adherence to substrates and the mechanical strength of images are improved. Thus, the prior art liquid developer has failed to be put to practical use.
The poor suitability for development and poor transferability described above are attributable to the insufficient control of the charge of toner particles. For improving image quality, use of a charge director, e.g., a dye or a metal soap, conventionally employed in liquid developers is indispensable.
The ordinary method for controlling the electrification of a toner by means of a charge director has been to add the charge director to a carrier liquid during developer preparation, as stated in two early reports by K. A. Metcalfe [J. Sci. Instrum., 32, 74 (1955) and Ibid., 33, 194 (1956)]. However, since such charge directors, e.g., a metal soap, generally lower the electrical resistance of carrier liquids considerably. It is therefore desirable from experience that for obtaining satisfactory image properties, a charge director be added to a carrier liquid in an amount as small as possible. On the other hand, in order to satisfactorily charge a toner, higher charge director concentrations in the carrier liquid are desirable; this is contrary to the above. The above-described method for toner charge control only by the post-addition of a charge director is disadvantageous in that actual copying operation using the thus-obtained liquid developer results in a change in the electrical resistance of the liquid developer from the initial value thereof due to the pressure of the unconsumed charge director remaining in the carrier liquid, so that stable formation of images is impossible. Moreover, in order for a metal soap or another compound to produce the effect of a charge director, it should form micelles in the carrier liquid used to thereby dissolve therein in some degree. However, most metal soaps and other charge director compounds are insoluble or sparingly soluble in the hydrocarbons, e.g., normal paraffins and isoparaffins, actually used advantageously as the carrier liquids of liquid developers, so that the desired charge control effect has not been obtained sufficiently.
In JP-A-58-152258 is proposed a process for producing a liquid developer, in which process a resin insoluble in solvents at room temperature is heated and melted along with a colorant and a charge control agent, e.g., a metal soap, in an appropriate solvent, this melt is cooled to room temperature to obtain a particulate toner, and the solvent is then replaced with the carrier liquid to be actually used. The developer obtained by this prior art process has an advantage that the carrier liquid can have reduced electrical resistance and relatively satisfactory images can be obtained initially. However, it has a drawback that it becomes impossible to obtain satisfactory images as copying is continued on a copier for practical use. Such image defects become severer especially when the toner concentration of the liquid developer varies considerably or when the temperature or humidity of the surrounding air changes. The reason for this may be as follows. In a developer which does not contain a charge director in the carrier liquid, no sites having the function of charge stabilization are present or, at the most, the water contained in the developer in a slight amount, hydrophilic impurities dissolved in the water, and other substances function as quasi charge stabilization sites. As a result, charge exchange between (1) toner particles and a charge director and between (2) toner particles and toner particles does not sufficiently occur and is influenced by variations in toner concentration, temperature, and humidity. Therefore, addition of a charge director, which is a substance used for accelerating charge exchange, to a liquid developer is indispensable.
Furthermore, most of the carrier liquids proposed so far for use in liquid developers are organic solvents having a high vapor pressure. Each carrier liquid hence has the following problems; (i) the carrier liquid vapor discharged at the time of fixing, etc. tends to cause environmental pollution, (ii) the carrier liquid vapor tends to catch fire, and (iii) after fixation to a substrate, e.g., paper, the carrier liquid remaining in the substrate gradually vaporizes to emit the odor of the solvent from the copy. Thus, the prior art carrier liquids are unable to sufficiently cope with present-day environmental regulations. To overcome these problems, a technique for preventing carrier vapor generation has, for example, been proposed, in which technique a high-molecular hydrocarbon substantially solid at ordinary temperature is used as a carrier and the development of an electrostatic latent image is conducted while the carrier is kept in a molten state using an appropriate heating means. In JP-A-2-6965 and JP-A-5-72820, for example, there is a description to the effect that developers containing a carrier which is solid at ordinary temperature are superior in maintenance and handleability to developers containing a carrier which is liquid at ordinary temperature. However, the developers described in these references have poor reliability because they have problems that the colloid stability thereof is impaired with heating or repeated thermal history of heating.fwdarw.cooling.fwdarw.heating in a copier for practical use and, as a result, aggregation or coagulation of toner particles and carrier viscosity increase occur undesirably during storage.